1. Field of the Invention
The present invention relates, in general, to fixtures for mounting tools used in locating, holding and assembling workpieces and, specifically, to fixtures for mounting tools used to assemble vehicles in production lines.
2. State of the Art
In modern automotive assembly operations, the major components of a vehicle body, such as the under body, side frames, and roof are initially tack welded together in a special fixture to insure dimensional accuracy. The partially assembled body is then conveyed by suitable means, such as ground or overhead conveyor, special purpose platform or framing buck, etc., to a respot or finish weld area where automatic welders, programmable robots, etc., make multiple welds and/or install additional panels and support members to the body. Such respot operations typically require multiple, successively arranged welding tools or robots to add the substantial number of welds needed to insure the structural integrity of the vehicle body.
In each individual weld station, the automatic tooling or robot must first accurately locate the body with respect to a known reference point before beginning the welding operation. Locating pins at each weld station are employed to engage reference apertures or "read holes" in the vehicle. In certain vehicle assembly operations, a lifter is located at each weld station to raise the locating pins into engagement with the "read holes" in the vehicle body and to lift the vehicle body off of the conveyor to a set working height for subsequent respot welding operations.
However, for efficient utilization of plant and tool resources, it is common for several different vehicle models or styles to be assembled on the same production line. Due to part geometry, each different vehicle model may require different tooling which, of necessity, is often located at different X and Y coordinates in each weld station. Due to the complexity of modern vehicles, completely separate, dedicated respot stations are often required for each different vehicle style. This increases tooling costs due to the need for separate welding equipment, robots, etc., as well as increasing plant floor usage requirements.
Additional increases in tooling costs are incurred during model changeover in which the tooling dedicated to a particular vehicle model or style is often discarded in favor of new tooling for a new model or for minor changes in the old model. In an attempt to reduce overall tooling costs, it is known to provide modular workpiece fixtures which include a plurality of apertures arranged in an X-Y grid pattern, such as that disclosed in U.S. Pat. No. 4,968,012. Tooling dedicated to a particular vehicle style or model is mounted on the modular frame in predetermined positions at desired X and Y coordinates. During model change or the revision of vehicle styles on a particular production line, the tooling can be repositioned on the modular fixture as needed, with only the portion of the tooling which contacts the vehicle body being revised for specific new features or vehicle models.
However, such modular tooling requires large surface plates with a substantial number of X-Y grid arranged apertures to provide the desired tooling repositioning capability. Such plates are heavy and require extensive precision machining for the X-Y grid arranged apertures thereby resulting in a high manufacturing cost.
It would be desirable to provide a tool mounting fixture particularly suited for vehicle assembly operations which substantially reduces tooling costs compared to previously devised tool mounting fixtures. It would also be desirable to provide a tool mounting fixture which is light in weight and has minimal precision machining requirements. It would also be desirable to provide a tool mounting fixture in which the tooling can be easily repositioned as necessary for different workpieces. Finally, it would be desirable to provide a tool mounting fixture utilizing many standard tooling components which require a minimal amount of replacement during changeover or the introduction of new models on a particular vehicle production line.